1. Field of the Invention
The present invention relates to a longitudinal-mode surface acoustic wave filter, a method of manufacturing a surface acoustic wave filter, and a communication device.
2. Related Art of the Invention
In recent years, surface acoustic wave filters have been widely used in mobile communication devices. Surface acoustic wave filters of a longitudinal mode type or a ladder type are used as a filter in a radio frequency (RF) stage. With the improvement in performance of communication devices such as portable telephones, there has been an increasing demand for reducing the loss and increasing the attenuation in surface acoustic wave filters.
A conventional longitudinal-mode surface acoustic wave filter will be described.
FIG. 12 shows a configuration of a conventional longitudinal-mode surface acoustic wave filter. As shown in FIG. 12, the surface acoustic wave filter has a piezoelectric substrate 801, first, second, and third interdigital transducer (IDT) electrodes 802, 803, and 804, and first and second reflector electrodes 805 and 806, the IDT electrodes and the reflector electrodes being formed on the substrate. The upper electrode fingers of each of the second and third IDT electrodes 803 and 804 is connected to an input terminal IN, while the lower electrode fingers of each of the second and third IDT electrodes 803 and 804 is grounded. The lower electrode fingers of the first IDT electrode 802 is connected to an output terminal OUT, while the upper electrode fingers of the first IDT electrode 802 is grounded. The distances between centers of adjacent pairs of the electrode fingers of the first, second, and third IDT electrodes 802, 803, and 804, represented by the distance indicated by P in FIG. 12 (hereinafter referred to as “pitch”), are equal to each other. The longitudinal-mode surface acoustic wave filter is thus constructed.
In the above-described surface acoustic wave filter, the electrode fingers are arranged with a constant pitch in order that the acoustic velocity of a surface acoustic wave be constant through the arrangement of the first, second, and third IDT electrodes 802, 803, and 804. In many instances, however, the number of electrode fingers of the first IDT electrode 802 and that of each of the second and third IDT electrodes 803 and 804 are set different from each other according to a design considering the bandwidth and impedance. Ordinarily, the surface acoustic wave filter is designed so that the number of electrode fingers of the first IDT electrode 802 is larger than that of each of the second and third IDT electrodes 803 and 804.
A longitudinal-mode surface acoustic wave filter has also been used which is designed so that the electrode fingers of each of electrodes have different pitches as shown in FIG. 13 to achieve a reduction in loss for example. The conventional longitudinal-mode surface acoustic wave filter shown in FIG. 13 is based on a design in which each IDT electrodes have an electrode finger pitch different from that in a main region.
Referring to FIG. 13, the surface acoustic wave filter has a piezoelectric substrate 1201, first, second, and third IDT electrodes 1202, 1203, and 1204, and first and second reflector electrodes 1205 and 1206, the IDT electrodes and the reflector electrodes being formed on the substrate. The upper electrode fingers of each of the second and third IDT electrodes 1203 and 1204 is connected to an input terminal IN, while the lower one of each of the second and third IDT electrodes 1203 and 1204 is grounded. The lower electrode fingers of the first IDT electrode 1202 is connected to an output terminal OUT, while the upper electrode fingers of the first IDT electrode 1202 is grounded.
Also, referring to FIG. 13, if the pitch in a region indicated by 1a in the first IDT electrode 1202 is P, P is ½ wavelength. If the pitch in a region indicated by 1b is P′, P′ is smaller than ½ wavelength. The pitch in a region indicated by 2a in the second IDT electrode 1203 is P, and P is ½ wavelength. The pitch in a region indicated by 2b is P′, and P′ is smaller than ½ wavelength. Similarly, the pitch in a region indicated by 3a in the third IDT electrode 1204 is P, and P is ½ wavelength. The pitch in a region indicated by 3b is P′, and P′ is smaller than ½ wavelength.
Thus, in each of the first IDT electrode 1202, the second IDT electrode 1203, and the third IDT electrode 1204, different electrode finger pitches are set between the electrode fingers in the same IDT electrodes.
Also in many instances relating to the arrangement shown in FIG. 13, the number of electrode fingers of the first IDT electrodes 1202 and that of each of the second and third IDT electrodes 1203 and 1204 are set different from each other according to a design considering the bandwidth and impedance. Ordinarily, the surface acoustic wave filter is designed so that the number of electrode fingers of the first IDT electrode 1202 is larger than that of each of the second and third IDT electrodes 1203 and 1204.
There is a problem in such a surface acoustic wave filter that there is a limit to improvement in filter characteristics in achieving a wide-band characteristic.